Amplifying system



Aug. 6, 1940. P. wEATHl-:Rs

AMPLIFYING SYSTEM Filed July 51, 1937 my.. cv m ||||\I.l|| mmm, im, 9F .uw n m mw u w N w w w R w E n@ Q Q/ Nm kmwm- HQ .N T R mkv fr@ m, nulli!!! |111.. ww

EN 7 .Q w+ mm1 n lm w+ @www N+ w\\w\ AM. w A W N@ v N H d H H NN, NM. M N E Q n. Q w s n QM- VIMNMWMWNIMNL ww n L Patented Aug. 6, 1940 AMPLiFYTNG SYSTEM Paul Weathera Audubon, N. J., assignor to Radio Corporation of America, a corporation o fDela.-

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Application `Iuly 31, 1937,;Serial No. 156,642

v 4 Claims.

` The present invention relates to amplifying systems for electric signals embodying a plurality of electric discharge ampliiier tubes in a series of amplifying stages. T

As is .well known, amplifiers of this character have inherent operating characteristics such as atendencyto introduce distortion in the signal reproduction. together with alternating current hum from associated power supply sources and `microphonic sounds resulting from mechanical disturbances of the tube elements if subjected to vibration from sounds or other sources.

In high gain amplifiers of theV character referred to as used in `audio frequency amplifiers,

l5: forr example, for radio receivers, public address systems and the like, the operating characteristics above referred to are considerably accentuated if at all present, and it is an object of the present invention to eliminate such disturbances in-anjelectric-discharge multi-stage amplier by providing an improved inverse feedback system therein embodying a minimum number of amplifying stages. l y

It is also lan object of the present invention to provide an improved electric discharge multistage amplifier wherein advantage is taken of in verse feedback or degenerative feedback to affect quiet operation of the amplifier without complicating the amplifier system by extended-feedz back loops.. I y

It is a further object of the present invention to provide Ya degenerative or inverse feedback multi-stage signal amplifier wherein the feedback energy may be derived from a signal stage.

Other objects and advantages of the invention will appear from theffollowing description When considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing,

Figure l is a schematic circuit diagram of a multi-stage signal amplier embodying the invention, and

Figure 2 is a similar schematic circuit diagram of a portion of the circuit of Fig. l showing a modification thereof. I

Referring to Fig. l, a signal ainplier is shown having three cascade connected amplifying stages and provided with amplifier tubes 5, Ii and'I in the rst, second and third stages, respectively.

The. amplifier input circuit is Aindicated at 8 and is provided with a ground or chassis return connection 9. The input circuit is coupled to the control grid I0 of the rst stage amplifier tube 5 lput-anode indicated at 34 which is coupled to an through a coupling capacitor II and grid resistor I2.

The tubek is of the indirectly heated type, the' cathode being indicated at I 3 and is provided with a self bias resistor I4 in the cathode connection to the chassis I5. The cathode resistor I4 is provided with a by-pass capacitor I6, the capacity value of which is sufficient to materially reduce the alternating current impedance across the resistor I4 at very high frequencies to provide a desirable high frequency rise if y desired.

The amplifier tube 5ris coupled for the transfer vof signals to the second stage 6 through any suitablercou'pling means such as the impedance coupling network indicated at I'I. As this type of coupling is well known, it is believed to require no further description except to indicate that the input grid for the tube 6 is returned to chassis through the grid resistor I8 and that'the bias for the grid .circuit through the resistor I8 is obtained from a cathode circuit resistory I9 in a similar manner to the rst stage 5.

In the second stage the tube is also of the indirectly heatedcathode type, the cathode being connected to ground o r chassis 2| through the Self bias resistor I9 which is also provided with a by-pass capacitor 22 having a capacity value hereinafter referred to.

The second stage amplier 6 is provided with an output circuit similar to that ofthe rst stage being of the limpedancecoupled type having an anode coupling resistor 23 and a coupling capacitor A2l! for transmitted amplified signals to the succeeding 'stage whichr is included within the dotted enclosure 25.

The third stage including the amplifier tube I is a degenerative feedback stage having ahigh potential grid or signal lead 26k connectedy with the signal grid 21 of the amplifier tube 'I and with the output coupling means of the preceding stage which, in the present example, is the coupling capacitor 24. The amplifier tube 'I is of the indirectly heated cathode type having the cathode 28 connected to'chassis'or ground 29 through a cathode or self bias' resistorA 30 provided with 4.5 an audio frequency by-pass capacitor 3| whereby amplifying signals from the preceding stages are applied between cathode and control grid through lthe connections referred to. A grid resistor32 connected between the high potential lead 26 orgrid and chassis provides biasing po- Itentialon ythe grid from the resistor 30.

The stage is also provided with an output circuit indicated at 33 in connection with the outoutput circuit 35, coupled thereto through a coupling capacitor 36 and an anode coupling resistor 37. Each of the anode circuits is provided with a suitable anode circuit lter for the B supply voltage as indicated at 38. The fact that such decoupling means is provided in each of the anode circuits, for individual biasing means is provided in each stage, renders the amplifier less subject to hum disturbances from the potential sources of supply which are usually of the alternating current type. However, the presence of any disturbances from microphonic vibration of the tube circuits or tube elements, the presence of alternating current hum in any of the circuits or distortion of the signal because of the circuit components may be reduced to any degree down to and including unity amplification by means of inverse or degenerative feedback connections indicated at 40 and 4| between the input and output circuits of the feedback stage and each of the cathode resistors of the preceding stages at the cathode ends thereof, as indicated. These connections are controlled by series variable resistors 42 and 43 and the coupling is confined to alternating or signal currents by series capacitors 44 and 45 introduced into the feedback circuit connections 40 and 4|'at suitable points such as adjacent to the feedback connections 46 and 4i with the input and output circuits respectively of one feedback amplifier stage.

The feediback connections are provided between the output circuit o-f the third stage adjacent to the high potential or anode portion thereof and the cathode of the next preceding stage amplifier tube, while in the input circuit of the feedback stage the connection is made with the cathode of the second preceding stage, whereby multiple and overlapping ydegeneration is provided.

The effect of this is to reduce the amplification in each of the stages to which degeneration is applied and in so applying the feedback, the distortion, hum and microphonic noises are reduced by the amount of the gain in the feedback loop. This may be reduced to a gain of unity. For eX- ample, if distortion within the amplifier is 2 percent with one stage having a gain of 10, if a stage is added also having a gain of 10 and degeneration is provided in a'10 to 1 ratio so that the original gain is restored, then the distortion is reduced to .0.2 percent. Thus, by the addition of a single stage to the two stage amplifier comprising the stages and 6 the gain in each of the rst two stages may be reduced by degeneration under control of the resistors 42 and 43 to any desired degree whereby the signal input at the input circuit 8 is amplified and reproduced at the output circuit 35 substantially free o-f distortion, hum and microphonic noises.

Considering the amplifier stages 6 and 'I as a two stage amplifier, it will be seen that the system shown provided for phase reversal or degeneration in each stage and that the feedback connection is simplified in that the feedback is made directly to the cathode of each stage and is thereby prevented from loading or otherwise affecting the grid circuit of each of the amplifying stages.

Referring to Fig. 2, it has been found that an amplifier comprising screen grid tubes indicated at 50 and 5|in. an amplifier channel may be connected to a feedback stage such as the stage 25 indicated in the block diagram in Fig. 2 through coupling resistors 5|a and 52 in the screen grid and cathode circuits, the `feedback being applied through the lead 4I directly to the screen grid 53 of the stage 50 and to the cathode 54 of the stage 5| to provide' degeneration in a plurality of stages with similar results as in the circuit of Fig. 1. The operation of the system and the stage 25 is .otherwise the same as in Fig. 1.

In accordance with the invention, it will be seen, therefore, that feedback energy is applied to one or more cascade connected amplifier stages from a succeeding stage through inverse feedback connections from that stage to the cathodes of the preceding stage amplifiers in the proper direction to cause degeneration and loss of ampliiication within the amplifier.

It will be observed that in the arrangement of Fig. 1 the feed-back of the stage 5 is derived from the output circuit of the stage E and the feedback of the stage G is `derived from the output of the stage both feedback circuits being connected to the cathode. The arrangement of Fig. 2 differs from that in Fig. 1 in that a common feedback from the output of the stage 1 is applied respectively, to the screen grid of the stage 50 and to the cathode of the stage 5|. The arrangement of Fig. 2 has the advantage in that by desirable proportions of coupling capacity and resistance any degree o-f low frequency or high frequency combination up to 6 db. per octave may be obtained. The feedback to stage 50 is limited at the low frequency end by `capacitor 55 and may be so proportioned with respect to resistor 56 that the low frequency rise may start at any desired frequency. 'Ihe feedback to stage 5I is by-passed by a capacitor 51. The capacitor 57 is so proportioned with respect tocathode resistor 58 that the high frequency rise may start at any desired frequency. The combined feedback to stages 50 and 5| are controlled by variable resistor 43.

Assuming that Rs equals 100,000, Xcss equals 100,000 at 200 cycles, feedback to stage 50 below 200 -cycles will be reduced by approximately 6 db. per octave, below 200 cycles.

If the resistance of 58 should be 2000 ohms and the capacity reactance of 51 should be 2000 ohms at 4000 cycles, then the rise will be approximately 6 db. per octave starting at 4000 cycles and continuing until the feedback is reduced substantially to zero. The latter, of course, does not take into account the degeneration in stage 5| due to plate currents of 5| flowing through cathode resistor 58.

It thus follows that the rise of the low frequency components in the stage 50 and the rise of the high frequency components of the stage 5| may be so combined by suitable adjustment of the constants of these circuits as to produce a desired frequency combination of the output circuit of the channel.

I claim as my invention:

1. In a multiple stage amplifier, the combination of means in one of said stages providing signal input and output circuits, a pre-ceding stage coupled to said input circuit and including an amplifier tube having a cathode electrode and means for applying alternating current degenerative feedback energy thereto, a second preceding stage coupled to the first preceding stage and including a tube having a screen grid electrode and means for applying alternating current degenerative feedback energy thereto, and means providing a single alternating current feedback path between said output circuit and said first and second-named means to provide degenerative feedback jointly to said cathode and said screen grid'electrode, thereby to pro- 75:

vide degenerative feedback in said preceding stages accumulatively without the intermediary of an interposed third stage, and separate means for modifying'the degenerative feedback to each of said electrodes in differing frequency ranges. e

2, In a multiple stage amplier, the combination of means in one of said stages providing signal input and output circuits, one of the preceding stages including an amplifier tube having a cathode electrode and means for applying alternating current feedback energy to said cathode electrode, another of said preceding stages including a tube having a screen grid electrode and means for applying alternating current feedback to said screen grid electrode, means providing Van alternating current feedback path between said output circuit and said first and second-named means-to provide degenerative feedback to said cathode and said screen grid electrode, thereby to provide degenerative feedback in said preceding stages accumulatively `Without the intermediary of an interposed third stage, and means in circuit with said cathode electrode fordecreasing the degenerative feedback thereto in response to the higher frequency components of signals transmitted through said amplifier.

3. In a multiple stage amplifier, the combination of means in one of said stages providing signal input and output circuits, one of the preceding stages including an amplifier tube having a cathode electrode and means for applying alternating current feedback energy to said cathode electrode, another of said preceding stages including a tube having a screen grid electro-de and means for applying alternating current feedback to said screen grid electrode, means providing an alternating current feedback path between said output circuit and said rst and sec.- i

ond-named means to provide degenerative feed- -back to said cathode and said screen gridelectrode, thereby to provide degenerative feedback in said preceding stages accumulatively without the intermediary of an interposed third stage, and means in circuit with said screen grid electrode for decreasing the degenerative feedback thereto in. response to the lower frequency components of signals transmitted through said ampliiier.

4; In a multiple stage amplifier, the combinaincluding a tube having a screen grid electrode and means for applying alternating current feedback to said screen grid electrode, means providing an alternating current feedback path between said output circuit and said first and second-named means to provide degenerative feedback to said cathode and said screen grid electrode, thereby to provide degenerative feedback in said preceding stages accumulatively without the intermediary of an interposed third stage, means connected respectively in circuit with said screen grid electrode and said cathode electrode for reducing the degenerative feedback thereto and accentuating the high and low frequency signal components, and means for jointly controlling the amount of said high and low frequency feedback.

PAUL WEA'II-IERS. 

